In the "The Revolt of Mother" by Mary Wilkins Freeman, why does Adoniram refuse to explain his decision to build a new barn to his wife?

Since the story is written from the limited point of view of Sarah Penn, readers don't have the ability to get inside Adoniram Penn's head to understand his motivations. However, from his actions and words, and from Sarah's words, readers can infer his motivations. The story begins with Sarah asking Adoniram why men are digging in the field, and Adoniram unwillingly divulges that they are building a barn, but not until expressing to Sarah that she should "'tend to your own affairs." He implies that running the farm, including making decisions about outbuildings, is his domain, not Sarah's. Throughout the play signs of the patriarchal rule in the Penn family are apparent, most specifically when Sarah Penn explains to her daughter that she will one day find out that "we know only what men-folks think we do ... an' how we ought to reckon men-folks in with Providence, an' not complain of what they do any more than we do of the weather." 

As much as Adoniram believes he has the authority and right to build the barn, he nevertheless certainly experiences twinges of conscience about his decision, which also probably motivates him to keep his wife in the dark about the barn. (His tears at the end of the story may indicate a heart soft enough to feel guilt.) He knows without Sarah's reminders that he promised her 40 years previously to build her a new house on the very place where he is now erecting his new barn. Yet when she confronts him on his failure to keep his promise, he replies, "I ain't got nothin' to say." One reason for his silence is that he cannot refute her allegations or her logic. Another is that he may feel guilty about his choices. But in the end, he employs silence as a defense against Sarah's influence so that he can continue to have his own way: "Her opponent employed that obstinate silence which makes eloquence futile with mocking echoes." 

From the clues in the story, readers can deduce that Adoniram refuses to explain his decision about the barn to Sarah because his reasons are indefensible given his prior promises, because he may feel somewhat guilty for having broken faith with his wife on the issue, because he wants his own way, and because the patriarchal system their marriage functions under allows him to do so.